1. Field of the Invention
The invention relates to a color correction for a (CRT) display device with a variable light transmission panel or coating.
2. Description of the Related Art
As described in U.S. Pat. No. 5,060,075, the perceived contrast of an image produced by a CRT display can be improved by using a variable light transmissivity panel adjacent to the CRT faceplate or by applying a variable transmissivity coating on the tube faceplate. The transmission of the panel or coating can be changed as function of the ambient lighting. Generally, the transmissivity of the panel decreases in response to an increasing ambient light intensity. For lower ambient light conditions, this will result in CRT tube with a higher transmissivity, resulting in a lower electron beam current which then result in a sharper picture and a lower energy consumption. Also, the gun characteristics can be changed. Normally the gun is optimised for larger currents. However, when the transmissivity of the panel is high and thus the electron beam current is low, the characteristics of the gun can be changed is such a way that the gun is optimal for lower beam currents. This can be done by changing the grid voltages. The transmissivity of a variable light transmission panel is controlled by applying a voltage to the panel.
As acknowledged in U.S. Pat. No. 5,847,858, corresponding to International Application No. WO-A-97/22906 and International Application No. WO-A-97/34192, corresponding to U.S. Pat. No. 5,859,735, some types of transmission panels are not an ideal neutral (grey) filter, but have a transmission T(.lambda.) which is not constant for the visible wavelength .lambda.. Also, the shape of transmission curve T(.lambda.) changes as a function of the applied voltage. In other words, the color of the panel is not grey, and changes as a function of the perceived transmissivity. If such a panel, for example, an electrochromic panel, is applied, the appearance of the colors produced by the tube-panel combination will change. The used electrochromic sample resulted in a bluish picture, because the transmissivity of red and green decreases faster than the transmissivity of blue, when the perceived transmissivity is decreased. To solve these problems, WO-A-97/22906 and WO-A-97/34192 propose to use adapted electrochromic elements for the transmission panel/coating.